The present invention relates to a dual tilt control system for a work vehicle, the system controlling a blade pivotably connected to a vehicle body by means of a pair of left and right tilt cylinders for pitch and tilt operations.
Conventionally, a work vehicle such as a bulldozer excavates, carries soil and levels the ground after excavation by means of a blade coupled to the front of a vehicle body through a pair of straight frames. One known technique of controlling the blade is as follows. A pair of left and right tilt cylinders (pitch cylinders), that is, double-acting hydraulic cylinders are provided between the blade and the vehicle body. When these tilt cylinders are driven simultaneously so as to extend or retract, the blade assumes a pitch dump posture (forward-tilting posture) or a pitch back posture (backward-tilting posture). When one of the tilt cylinders is driven so as to extend or retract with the other tilt cylinder remaining at rest, the blade assumes such a posture (right-tilting posture or left-tilting posture) that its right edge or its left edge tilts downward.
There is also disclosed what is called a dual tilt control system (e.g., in Japanese Patent Publication No. 63-501228 (1988)) which drives one of the tilt cylinders for extension while driving the other tilt cylinder for retraction, thereby obtaining an increased velocity at which the blade is actuated to tilt.
The conventional dual tilt control system is explained further with reference to a hydraulic circuit diagram shown in FIG. 5. A dual tilt control system 100 controls a right tilt cylinder 101 and a left tilt cylinder 102, which connect the blade (not shown) and the vehicle body (not shown), for pitch and tilt operations.
In the dual tilt control system 100, a directional control valve 105 is connected to a hydraulic pump 103 and to a tank 104 and has control ports 106, 107 connected to respective valve ports 110, 111 of a two-position switching type pitch control valve 108 and a two-position switching type tilt control valve 109, respectively. The pitch control valve 108 also has a cylinder port 112 connected to a bottom oil chamber 102a of the left tilt cylinder 102 and cylinder ports 113, 114 connected to a bottom oil chamber 101a and a head oil chamber 101b of the right tilt cylinder 101, respectively. The tilt control valve 109 also has a cylinder port 115 connected to a head oil chamber 102b of the left tilt cylinder 102 and cylinder ports 116, 117 connected to the bottom oil chamber 101a and the head oil chamber 101b of the right tilt cylinder 101, respectively. The right tilt cylinder 101 is of larger diameter than the left tilt cylinder 102 so that the head oil chamber 101b of the right tilt cylinder 101 and the bottom oil chamber 102a of the left tilt cylinder 102 can be of equal cross-sectional area. When an operating lever (not shown) is manipulated, the directional control valve 105, the pitch control valve 108 and the tilt control valve 109 are actuated upon receipt of electric signals or pilot pressure signals.
In the case of pitch dump, the directional control valve 105 and the tilt control valve 109 are switched to their respective positions A, while the pitch control valve 108 is switched to position B. Pressure oil discharged from the hydraulic pump 103 is thus supplied through the control port 106 of the directional control valve 105, the valve port 110 and the cylinder port 113 of the pitch control valve 108 to the bottom oil chamber 101a of the right tilt cylinder 101, thereby actuating the right tilt cylinder 101 in an extending direction. The pressure oil from the head oil chamber 101b of the right tilt cylinder 101 is supplied through the cylinder ports 114, 112 of the pitch control valve 108 to the bottom oil chamber 102a of the left tilt cylinder 102, thereby also actuating the left tilt cylinder 102 in the extending direction. The pressure oil within the head oil chamber 102b of the left tilt cylinder 102 is exhausted through the cylinder port 115, the valve port 111 of the tilt control valve 109 and the control port 107 of the directional control valve 105 to the tank 104. The right and left tilt cylinders 101, 102 are thus extended simultaneously at an equal velocity, and consequently, the blade performs pitch dump (tilts forward). In the case of pitch back, the directional control valve 105 is switched to position B with the pitch control valve 108 and the tilt control valve 109 remaining in position B and position A, respectively. The pressure oil thus flows reversely to the direction described above, so that the right and left tilt cylinders 101, 102 are retracted simultaneously at an equal velocity. Consequently, the blade performs pitch back (tilts backward).
In the case of dual tilting to the right, the directional control valve 105 and the pitch control valve 108 are switched to their respective positions A, while the tilt control valve 109 is switched to position B. The pressure oil discharged from the hydraulic pump 103 is thus supplied through the control port 106 of the directional control valve 105, the valve port 110 and the cylinder port 112 of the pitch control valve 108 to the bottom oil chamber 102a of the left tilt cylinder 102, thereby actuating the left tilt cylinder 102 in the extending direction. The pressure oil from the head oil chamber 102b of the left tilt cylinder 102 is supplied through the cylinder ports 115, 117 of the tilt control valve 109 to the head oil chamber 101b of the right tilt cylinder 101, thereby actuating the right tilt cylinder 101 in a retracting direction. The pressure oil within the bottom oil chamber 101a of the right tilt cylinder 101 is exhausted through the cylinder port 116, the valve port 111 of the tilt control valve 109 and the control port 107 of the directional control valve 105 to the tank 104. The extension of the left tilt cylinder 102 and the retraction of the right tilt cylinder 101 are thus carried out simultaneously, and consequently, the blade tilts to the right at a high velocity (substantially double the normal velocity). In the case of dual tilting to the left, the directional control valve 105 is switched to position B with the pitch control valve 108 and the tilt control valve 109 remaining in position A and position B, respectively. The pressure oil thus flows reversely to the direction described above, so that the extension of the right tilt cylinder 101 and the retraction of the left tilt cylinder 102 are carried out simultaneously. Consequently, the blade tilts to the left at a high velocity (substantially double the normal velocity).
In the case of single tilting to the right, the directional control valve 105, the pitch control valve 108 and the tilt control valve 109 are switched to their respective positions A. The pressure oil discharged from the hydraulic pump 103 is thus supplied through the control port 106 of the directional control valve 105, the valve port 110, the cylinder port 112 of the pitch control valve 108 to the bottom oil chamber 102a of the left tilt cylinder 102, thereby actuating the left tilt cylinder 102 in the extending direction. The pressure oil from the head oil chamber 102b of the left tilt cylinder 102 is exhausted through the cylinder port 115, the valve port 111 of the tilt control valve 109 and the control port 107 of the directional control valve 105 to the tank 104. Only the extension of the left tilt cylinder 102 is thus carried out with the right tilt cylinder 101 remaining at rest, and consequently, the blade tilts to the right at a normal velocity (low velocity). In the case of single tilting to the left, the directional control valve 105 is switched to position B with the pitch control valve 108 and the tilt control valve 109 remaining in their respective positions A. The pressure oil thus flows reversely to the direction described above, so that only the retraction of the left tilt cylinder 102 is carried out with the right tilt cylinder 101 remaining at rest. Consequently, the blade tilts to the left at a normal velocity (low velocity).
The foregoing conventional dual tilt control system, however, has an inevitable problem of increased cost because of the following reason. The switching valves such as the pitch control valve 108 and the tilt control valve 109 are provided in a main hydraulic circuit intended for the tilt operation. This requires that the pitch control valve 108 and tilt control valve 109 be adapted to high pressure and high flow rates for use.
The foregoing conventional system also requires that the head oil chamber 101b of the right tilt cylinder 101 and the bottom oil chamber 102a of the left tilt cylinder 102 be of equal cross-sectional area for the pitch operation. Consequently, two types of cylinders that have bores of different diameters must be prepared, thus causing the cost to increase further.
Moreover, the foregoing hydraulic circuit has the right tilt cylinder 101 and the left tilt cylinder 102 that are arranged in series, so that a piston valve needs to be annexed to each of the cylinders 101, 102 to relieve the pressure oil when one of the cylinders reaches a stroke end first. This series arrangement also presents a problem that it cannot yield large force during the pitch and dual tilt operations.
The present invention addresses the problems discussed above and aims to provide a dual tilt control system for a work vehicle, the system being capable of using a switching valve adapted to low pressure and low flow rates and right and left cylinders of the same diameter and yielding large force during the pitch and dual tilt operations, and requiring no piston valve.
The above object can be accomplished by a dual tilt control system for a work vehicle according to a first aspect of the present invention, the dual tilt control system comprising: a blade pivotably connected to a vehicle body; a pair of left and right tilt cylinders disposed between the blade and the vehicle body; and tilt cylinder controlling means for controlling actuation of the tilt cylinders,
the tilt cylinder controlling means comprising a hydraulic circuit comprising:
a left tilt cylinder driving hydraulic circuit for supplying pressure oil from a hydraulic pump to the left tilt cylinder, the left tilt cylinder driving hydraulic circuit comprising a left tilt cylinder actuation switching valve;
a right tilt cylinder driving hydraulic circuit for supplying the pressure oil from the hydraulic pump to the right tilt cylinder, the right tilt cylinder driving hydraulic circuit comprising a right tilt cylinder actuation switching valve; and
a pilot pressure signal circuit for controlling the left tilt cylinder actuation switching valve and the right tilt cylinder actuation switching valve, the pilot pressure signal circuit comprising a pilot switching valve for switching the blade between a pitch operation, a single tilt operation and a dual tilt operation by means of the left and right tilt cylinders,
wherein the left tilt cylinder driving hydraulic circuit and the right tilt cylinder driving hydraulic circuit are arranged in parallel.
According to the present invention, the pilot switching valve is provided in the pilot pressure signal circuit that controls the left tilt cylinder actuation switching valve and the right tilt cylinder actuation switching valve, and through actuation of the pilot switching valve, the blade is switched between the pitch operation, single tilt operation and dual tilt operation by means of the left and right tilt cylinders, so that the pilot switching valve can simply be adapted to low pressure and low flow rates, thus becoming low-cost. Moreover, the left tilt cylinder driving hydraulic circuit and the right tilt cylinder driving hydraulic circuit are arranged in parallel, so that the left and right tilt cylinders of the same diameter can find use, and large hydraulic force can be applied during the pitch operation and the dual tilt operation to yield large force. Furthermore, there is no need to annex a piston valve for relieving the pressure oil at a stroke end.
According to a second aspect of the present invention, it is preferable that a pressure compensating valve is provided on either an upstream side or a downstream side of each of the left and right tilt cylinder actuation switching valves so that differential pressure between the upstream and downstream sides of each of the left and right tilt cylinder actuation switching valves can stabilize. Even in cases where there is a difference in load pressure between the left and right tilt cylinders, the pressure compensating valves automatically adjust respective spool opening areas of the actuation switching valves, respectively so that a velocity at which the left tilt cylinder is actuated can always be equal to a velocity at which the right tilt cylinder is actuated. A displacement time of the left part of the blade can thus be equal to a displacement time of the right part of the blade during the pitch operation or the dual tilt operation, and these displacement times can be short, thus advantageously facilitating efficient operation.
According to a third aspect of the present invention that is based on the first aspect or the second aspect, it is preferable that each of the left and right tilt cylinder actuation switching valves has a ratio of a main spool opening area, which is used for supplying a head oil chamber of each of the left and right tilt cylinders with the pressure oil, to a main spool opening area, which is used for supplying a bottom oil chamber of each of the left and right tilt cylinders with the pressure oil, that is set equal to a ratio between respective cross-sectional areas of a head oil chamber and a bottom oil chamber of each of the tilt cylinders. Thus, a velocity at which pitch damp caused by the operation of each of the tilt cylinders is carried out can be equal to a velocity at which pitch back caused by the operation of each of the tilt cylinders is carried out, thus allowing more efficient operation.